Sensitivity Studies

James Done

NCAR Earth System LaboratoryNational Center for Atmospheric Research

NCAR is Sponsored by NSF and this work is partially supported by the Willis Research Network and the Research Program to Secure Energy for

America

Climate Change Scenario

• Emissions scenario uncertainty

• Climate change uncertainty

Choice of Base Period

• The 20-year period of 1975-1994 was chosen based on:- need to smooth out influences of El Niño - quality of data in early period is poor - exclude apparent climate shift in 1995

• Caution: Multi-decadal variability, climate trends and shifts.- 1975-1994 had an average 8.9 TCs/yr- 1995-2005 had an average 14.3 TCs/yr

• Plan to test another base period.

Multi-Decadal Variability

• Global Model has a signal on multi-decadal timescales although the physical realism is in question.

• Can not expect global model variability to be in correct phase with real climate.

• NRCM time-slice approach shows 11-year windows into the full period.

• Complimentary downscaling approaches cover the full period.

Physics ParameterizationsNRCM physics choices based on:• Seasonal sensitivity tests. Model vs observed spatial distributions of tropical rainfall.• A 10-year simulation of current climate driven by global reanalysis data showed reasonable global distributions of tropical cyclones.

Observation1995 - 2000

Regional Model1995 - 2000

NRCM ResolutionSimulations of the 2005 North Atlantic Hurricane Season

• 36 km based on available resources on NCAR’s bluefire and the success of previous simulations of tropical cyclones in current climate at 36 km• Currently running at 12 km at Argonne National Lab

36 km18 Storms

12 km29 Storms

27 Storms Observed

Internal Variability (IV)

• Simulations are sensitive to small perturbations in the initial state due to non-linear dynamic and thermodynamic relations and multi-scale feedbacks that govern the climate system.

• Puts a predictability limit on details of weather to about 2 weeks.

• Open question: What is the impact of IV on tropical cyclones? - could be important for large regional domains.

Aim: Distinguish the reproducible climate change signal associated with external forcing from the component due to IV.

Sensitivity to Initial Conditions• 10 member ensemble• Cold starts, 10 days apart, from 1st Jan 2047 to 1st Apr 2047

Total column water vapor forSeptember 2047.(top) Cold start 1st Jan 2045(middle) Cold start 1st Jan 2047(bottom) Cold start 1st Apr 2047

Sensitivity to Initial Conditions

Tropical Cyclones in 2047:Average number 15.3Standard deviation 2.0Range 13 - 20variance/mean = 0.26

• Currently exploring methods to quantify IV and consequences for our climate variability and change results.

• Outliers suggest IV may be non-Gaussian.

Impact of Spectral Nudging2005 Aug-Sept-Oct(mm dd)

Observation15 Storms

Impact of Spectral Nudging2005 Aug-Sept-Oct(mm dd)

Observation15 Storms

Spectral Nudging9 Storms

Control12 Storms

SummaryA thorough investigation into the sensitivities of our modeling system is essential for assessing confidence in our climate variability and change results.

Source of Uncertainty ApproachClimate change scenario 3 emissions scenariosBase period Tests are plannedMulti-decadal variability 3 complimentary downscaling methodsPhysics Previous experienceResolution 2 resolutionsInternal variability Comprehensive

assessment underwaySpectral nudging Choose not to apply